It must not be possible to duplicate documents. Modern documents are therefore provided with security elements, which for instance, render information as to the authenticity of the protected document visible even to an untrained layman through diffraction of the ambient light. Such a document including an optical-diffraction type security element and a process for its production is described in U.S. Pat. No. 4,184,700 which is equivalent to Swiss Patent specification CH-PS No. 594,936.
Diffraction of light is illustratively obtained from optical coatings with embossed grid structures which are straight-line relief structures in their simplest form. The diffraction properties of these relief structures are determined, among other factors, by the spatial frequency, i.e. the number of lines per millimeter, by the cross-sectional form of the relief structure and by the differences in height within the relief structure as well as by the orientation of the relief structure on the document.
The U.S. Pat. No. 4,892,385 describes a security element with a diffracting structure which is composed of surface portions with varying structural parameters. Each individual surface portion is at least large enough so that light which falls on the surface portion is split into its color components in a characteristic manner and is recreated at a distance of 30 cm separate bundles of characteristic color are created at a solid angle of at least 2 milliradians, i.e. approximately 7 minutes of a degree of arc. These surface portions are therefore easy to recognize with the naked eye and stand out from adjoining surface portions through contrasting colors, for instance.
If the relief structure grid parameters change continuously, or in small steps within the diffracting structure from surface portion to surface portion, color impressions moving along a predetermined path are created on the document for the eye of the viewer when the position of the document in relation to the source of light and to the naked eye is changed. Documents of this type are described in EP (European patent) No. 105 099 which is equivalent to U.S. Pat. No. 4,568,141.
These security elements can be produced economically by shaping a layer of synthetic material with an embossing stamp capable of being heated and supporting the negative of the security element, said negative having been galvanically formed by a master structure.
The manner of producing this master structure is known from EP 169 326 which is equivalent to U.S. Pat. No. 4,761,253. A layer of synthetic material is elastically shaped in a small, circular zone by a matrix which supports a predetermined relief structure. The surface facing the matrix is heated locally by means of localized radiation until the synthetic material is softened in a surface area limited by the localized radiation and the relief structure is transferred to the surface. A master structure produced in this manner can comprise of a great number of surface portions which vary in their diffraction properties from adjoining surface portions. Forgery is made much more difficult by the great number of surface portions diffracting light in different ways.
The object of the instant invention is to create a document of the type mentioned initially, as well as a device for its production, with a structure which can be copied or forged only at great expense, which has lines and surface portions visible without any device through visual observation and with additional graphic features that cannot be seen without a (magnifying) device.